Apparatus for crucible-free zone melting of semiconductor rods



Sept. 11, 1962 w. KELLER ETAL APPARATUS FOR CRUCIBLE-FREE ZONE MELTING OF SEMICONDUCTOR RODS Filed April 20, 1960 United. States .filice 3,053,918 APPARATUS FOR CRUCIBLE-FREE ZONE MELT- ING OF SEMICONDUCTOR RODS Wolfgang Keller, Pretzfeld, Upper Franconia, and Reimer Emeis, Ebermannstadt, Upper Frauconia, Germany, assignors to Siemens-Schuckertwerke Aktiengesellschaft, Berlin-Siemensstadt, Germany, a corporation of Germany Filed Apr. 20, 1960, Ser. No. 23,535 Claims priority, application Germany May 14, 1959 2 Claims. (Cl. 13-1) Our invention particularly relates to apparatus for the zone melting of semiconductor rods by the so-called floating-zone method.

In the production of hyperpure monocrystalline semiconductor rods, particularly silicon rods, by floating-zone melting, the rod and the heating device for producing the molten zone are preferably mounted in a vessel, for example within an evacuated metal bell, the rod being held at both ends in vertical position. The rod is inductively heated by means of a heater coil which surrounds the semiconductor rod and is energized by high-frequency alternating current. The melting is limited to a narrow zone, and this zone is repeatedly passed in a single direction along the length of the rod. In general, the inductive heater coil heretofore has had the shape of a cylinder, and its turns, depending upon the coil diameter, are axially spaced from each other the smallest feasible amount in order to keep the height of the coil and thus the height of the melting zone as small as possible.

Such an inductive heater coil, however, has the disadvantage of obscuring the view onto the melting zone located within the turns of the coil. For best control of the heating power supplied to the coil, it is desirable to permit a continuous visual observation of the shape exhibited by the molten zone because overheating of the melt is noticeable from a change of that shape. Further-, more, the precipitation of evaporated silicon may electrically short the winding turns when the slight spacing between the turns is bridged by the silicon precipitate. In the event of a disturbance, the molten zone may freeze very rapidly and the resulting jump in volume of the crystal during solidification may cause protuberances to grow out of the rod. Due to the slight spacing between the coil and the molten zone and between the individual turns of the coil, such protuberances then tend to grow a further extent when the operation is continued. The short circuit thus occurring prevents the melting zone from being reheated from frozen to molten condition, and the coil must then be mechanically separated from the rod. This is an intricate and time consuming operation, and may easily damage the coil and result in contaminating the semiconductor material by impurities.

It is an object of our invention to avoid such disadvantages.

To this end, and according to our invention, an apparatus for the crucible-free zone melting of semiconductor rods, operating generally as described above, has its ring-shaped high-frequency inductance coil designed as a fiat coil structure having a single layer of spirally wound turns.

Such a flat spiral coil has the advantage that its magnetic alternating field is more sharply limited and has a flattened shape corresponding to the shape of the coil, whereby the height of the melting zone can be kept correspondingly small. This is because the coil has an effect similar to that of a concentrator inasmuch as the magnetic field of the outer Winding turns is pressed toward the center point of the coil and thus toward the rod. As mentioned, the flat shape of the coil also permits keeping the molten zone under continuous visual observation because the view is not obstructed by axially sequential winding turns.

The foregoing and other objects, advantages and features of our invention will be apparent from the embodiment of zone melting apparatus illustrated, by way of example, in the accompanying drawing in which:

FIG. 1 is a partly sectional top view, FIG. 2 a sectional side view of an inductive zone-melting heating device according to the invention, and FIG. 3 is a vertical sectional view of complete crucible-free zone-melting apparatus of the instant invention.

The apparatus comprises an inductive heater coil 2 consisting of good-conducting metal tubing, such as a pipe of silver or heavily silver-plated copper. The coil has the shape of a spiral and comprises a number of turns, for example, three turns as illustrated. The tubular ends 5 of the coil are firmly joined, by soldering, with a prismatic head piece 6 which is rigidly secured by means of screw nuts 7 to a terminal piece 8, a gasket 9 vof synthetic material being provided between pieces 6 and 8 to provide for a liquid-tight seal. Current is supplied to the two terminal pieces 8 by means of respective fiat bars 10 which are rigidly joined with a coil transporting device 12. Joined with the bars 10 are respective tubes 11 for passing cooling water through the coil 2 during operation of the device. The pair of current supply bars 10 has a curved shape. A heat shield 13, consisting of heat resistant metal, is mounted along the transporting device 12 for protecting it from the heat radiating from the molten zone. The current supply bars 10 preferably consist of copper.

FIG. 2 shows a cross-section along the line II--II in FIG. 1, passing through the axis of the semiconductor rod 3 and also through the coil 2 and through a portion of the current supply devices. The molten zone produced by the action of coil 2 is denoted by 4. Each of the two coil ends 5 is so fastened to its respective head piece 6 that the tube ends protrude through the head piece and somewhat beyond the other side thereof. The protruding tube end is inserted in a mating bore of the terminal piece 8. The tubes 11 for supplying cooling water are soldered into the bores of the two head pieces 8.

According to FIG. 3, the device comprises a vessel composed of bell 31 of relatively great width which is vacuum-tightly seated upon a bottom plate 32. The bell 31 may consist of steel or other sheet metal and is preferably provided with an observation window 31a of glass. The processing space within the vessel can be evacuated through a nipple 31b to be connected to a vacuum pump. If desired, an inert gas may also be supplied through the nipple. Mounted on the bottom plate 32 of the vessel is a frame structure 21 which carries a holder 22 vertically above another holder 23 that is mounted on the bottom plate 32 and may be fastened to a shaft 25 passing through a seal 33 in the bottom plate in order to permit imparting rotation to the semiconductor material 24 during the zone-melting operation, as is desirable for some purposes.

The semiconductor rod 24, for example of silicon, is held in and between the two holders 22 and 23 and is surrounded by an inductive heating coil 26. During operation of the device, with rod 24 inserted as shown and the processing space evacuated, the coil 26 is energized by electric current and causes a narrow horizontal zone of the rod 24 to melt, while being slowly shifted upwardly or downwardly along the rod thus gradually passing the melted zone through the entire body for the purpose of purifying it and/ or converting it to monocrystalline condition.

Coolant, such as water, passes through the turns of the heater coil. The entrance and exit of the flowing coolant are indicated in FIG. 3 by arrows 39.

Structures such as that described in a coassigned application of Schweickert et al., Serial No. 736,387, filed May 19, 1958, may also be used.

The construction of the inductance coil as a flat spiral structure has the further advantage of preventing the danger of contamination of the semiconductor rod by any particles that may scale off the surface or oil the coating of the coil. This advantage arises because the only coil area that faces the melting zone is virtually only the inner side of the single inner turn, and this area is slight in comparison with the inner area of the cylindrical heater coils heretofore used for such purposes. Furthermore, the cleaning of the flat spiral coil is facilitated because the winding turns are readily accessible from the top as well as from the bottom sides.

By virtue of the sharply limited magnetic field in the melting zone, the flat coil reacts much more favorably to any variations in cross-section of the melting zone, in comparison with the cylindrical coils, when the heating current is regulated for constancy in dependence upon the electric resistance of the melting zone. This electric resistance is varied by increasing or decreasing the mutual axial spacing of the two holders to which the rod is secured at its two ends.

We claim:

1. Apparatus for crucible-free floating zone melting in vacuum of rod-shaped semiconductor material, comprising an enclosing evacuated vessel, axially-spaced holders in the vessel for securing the rod substantially vertically therebetween, an annular electric heating member encircling the axis of the holders and being axially short with respect to the space between said holder members, said heating member and said holder members being mounted for relative axial displacement, the annular heating member comprising an inductive heating coil energized by high-frequency electric alternating current, the coil being a flat planar, hollow tubular coil having, in the axial direction, a single layer of spirally wound turns, connections to said tubular coil for passing a cooling fluid therethrough and for passing said electric current therethrough, said vessel being formed to permit sighting therethrough at the molten zone formed in the semiconductor material.

2. Apparatus for crucible-free floating zone melting of rod-shaped semiconductor material, comprising an enclosing evacuable vessel, axially-spaced holders in the vessel for securing the rod substantially vertically therebetween, an annular electric heating member encircling the axis of the holders and being axially short with respect to the space between said holder members, said heating member and said holder members being mounted for relative axial displacement, the annular heating member comprising an inductive heating coil energized by high-frequency electric alternating current, the coil being a flat planar, hollow tubular coil having, in the axial direction, a single layer of spirally wound turns, connections to said tubular coil for passing a cooling fluid therethrough and for passing said electric current therethrough, said vessel being formed to permit sighting therethrough at the molten zone formed in the semiconductor material.

References Cited in the file of this patent UNITED STATES PATENTS 2,664,496 Brace Dec. 29, 1953 2,686,864 Wroughton et al Aug. 17, 1954 2,792,317 Davis May 14, 1957 2,870,309 Capita Jan. 20, 1959 

